Geological outline of the Alps

The Alps were developed from the Cretaceous onwards by subduction of a Mesozoic ocean and collision between the Adriatic (Austroalpine-Southalpine) and European (Penninic-Helvetic) continental margins.The Austroalpine-Penninic wedge is the core of the collisional belt, a fossil subduction complex which floats on the European lower plate. It consists of continental and minor oceanic nappes and is marked by a blueschist-to-eclogite-facies imprint of Cretaceous-Eocene age, followed by a Barrovian overprint. The collisional wedge was later accreted by the Helvetic basement and cover units and indented by the Southalpine lithosphere, which in turn was deformed as an antithetic fold-and-thrust belt.     

Geochemistry and palaeotectonic setting of amphibolites from the Western Tatra Mountains,southern Poland

Amphibolites from the crystalline basement of the Western Tatra Mountains, which are found as small lenses within migmatitic gneisses and mica schists, were formed during pre‐ or early Variscan amphibolite‐facies metamorphic events, and subsequently intruded by the post‐metamorphic Variscan Tatra Granite. The amphibolites occur in both the upper and lower metamorphic complexes, which are separated by a major subhorizontal shear zone in the Western Tatra Mountains. The amphibolites can be divided into three types: massive, striped and garnetiferous. The striped and massive amphibolites, concordant with a dominant S₁ foliation, and the garnet amphibolites, which cross‐cut the S₁ banding in the gneisses, were all originally intrusive dolerites. The striped amphibolites (consisting primarily of hornblende, andesine and quartz), and later, cross‐cutting garnet‐hornblende‐andesine‐quartz‐bearing amphibolites, predominate in the lower part of the dominantly migmatitic upper complex, and are exposed mainly on the ridges. The massive amphibolites, which contain a similar mineral assemblage, mainly occur in the usual unmigmatized lower structural unit. Chemical studies indicate that three amphibolite suites are present, which probably originated as a series of enriched tholeiites, similar to more recent plume‐influenced magmas, which were derived by partial melting of a spinel lherzolite with primitive mantle composition and compositionally slightly modified by crustal contamination. The amphibolites were intruded as dolerites into clastic sediments which had accumulated in an extensional basin floored by attenuated continental crust, a situation similar to that of amphibolites found in metamorphic complexes within the Variscan belt, e.g. in the Orlica–Snieznik area of the Sudetes, where amphibolites chemically similar to those in the Western Tatra also occur. Copyright © 2000 John Wiley & Sons, Ltd.     

Post-collisional tectonics in the Northern Cottian Alps (Italian Western Alps)

Field mapping and structural analysis have allowed us to characterise the fault geometry and the post-metamorphic tectonics of an area located in the Northern Cottian Alps (inner Western Alps). Two main faulting stages were distinguished here. The first (Oligocene?-Early Miocene) is related to the development of an E–W-striking left-normal shear zone. This shear zone is interpreted as an antithetical of two regional, N–S right-lateral structures: the Col del Lis-Trana Deformation Zone (LTZ) and the Colle delle Finestre Deformation Zone (CFZ). The second faulting stage (post-Early Miocene) is related mainly to the development of N–S normal faults, coeval with the extensional reactivation of the LTZ and the CFZ. We discuss this kinematic evolution in the framework of the geodynamic evolution of the Western Alps.     

Petrochemical Study of Lherzolitic Rocks from the Western Alps

An integrated geologic and petrochemical comparative study offive peridotite complexes in the western Alps has been undertaken.Investigated bodies are exposed at Alpe Arami in southern Switzerland,and at Finero, Balmuccia, Baldissero and Lanzo in northwesternItaly. The Alpe Arami mass has been tectonically emplaced withinthe Lepontine gneiss terrane of the in part subducted, morenortherly European lithospheric plate, whereas the other fourultramafic masses represent original portions of the non-subductedSouth Alpine plate. Eighty samples were examined petrographically. Most are lherzolites,but a few are clinopyroxene-bearing harzburgites, olivine websteritesor clinopyroxenite. Alpe Arami peridotites locally contain primarygarnet (± minor spinel); in contrast, the South Alpinelherzolites are spinel bearing, and in addition, the Lanzo massifcontains widespread plagioclase. All masses display strain effectssuch as bent lamellae in pyroxenes, gradational or sectoraloptical extinction, and minor recrystallization. Twenty-fourbulk XRF analyses demonstrate that the investigated rocks fairlyclosely match pyrolite composition, but are slightly impoverishedin alumina. The normative olivines of all analyzed specimenshave Fa contents ranging between 8 and 10 mole per cent. Electronmicroprobe analyses for 26 olivines, 27 orthopyroxenes, 23 calcicpyroxenes, three garnets, 18 spinels, three plagioclases, 13calcic amphiboles, two chlorites and two phlogopites are presented.Phases in a particular sample are remarkably homogeneous, anobservation consistent with an inferred close approach to chemicalequilibrium. Use of various two-pyroxene geothermometers allows the followingassignments of grand average apparent temperatures for the fiveperidotite complexes: Alpe Arami, 966±78°C; Finero,893 ± 94°C; Balmuccia, 973 ± 50°C; Baldissero,1002 ± 37°C; and Lanzo, 1069 ± 85°C. Pressureestimates, determined using the lherzolitic petrogenetic grid,Al₂O₃^cpx and Al₂O₃^opx isopleths, involve large uncertainties.The Alpe Arami ultramafic body evidently crystallized at a pressureof 40 ± 10 kilobars, the nominal value depending on themethod of computation. In contrast, the South Alpine spinellherzolites seem to have last equilibrated at pressures of approximately5–20 kb. The deep upper-mantle source region of the AlpeArami body apparently was subjected to a subcontinental-typegeothermal gradient, consistent with solid-state rise of thismass and Late Alpine tectonic insertion in the old Lepontinesialic crust of the European lithospheric plate. The mechanismand driving force of this process remain obscure. In contrast,the investigated spinel peridotite complexes of the South Alpinelithospheric plate seem to have been derived from shallow, uppermostmantle sections characterized by oceanic geothermal gradients.For this reason it is conjectured that these massifs were derivedfrom the northern margin of the southern plate where continentalcrust evidently was moderately thin or absent. Their upliftand overthrusting involved P-T paths which in most cases alloweddecompression partial recrystallization and incipient fusion,the latter indicated by the presence of transecting mafic dikesand segregations associated with the peridotites. The Finerobody appears to have reached its present position at the baseof the South Alpine continental crust in pre-Mesozoic time,whereas the Lanzo complex was tectonically involved in the EarlyAlpine orogeny.     

On the significance of aragonite occurrences in the Western Alps

Aragonite occurrences from two areas of the Western Alps are described. It is shown that aragonite has been formed under blueschist metamorphic conditions in the Western Vanoise, while it has been precipitated under sub-surface conditions in the Queyras region. An uplift (P, T) path of the aragonite-bearing rocks of the Western Vanoise is constructed using two independent methods: (1) temperature estimates using the Sr⁺⁺ content of aragonite in the successive veins and (2) the kinetics of the aragonitecalcite solid-state transition. The uplift (P, T) path has an unusual shape with an important temperature decrease (100° to 150° C for a pressure decrease of 0.2 GPa) following blueschists metamorphism (P=0.7 GPa, T=300° C). Thermal models show that this unusual (P, T) path of the Western Vanoise can be explained if one maintains a low temperature (between 125° and 175° C) at the base of the tectonic unit containing the aragonite-bearing rocks during part of its burial history, followed by the whole of its exhumation. A tectonic scenario is proposed to account for the observed and modelled (P, T) path.     

Garnet–omphacite–phengite thermobarometry of eclogites from the coesite-bearing unit of the southern Dora-Maira Massif, Western Alps

Using relevant geothermobarometric methods, P–T-data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh-pressure (UHP) metamorphic coesite-bearing Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif. The mineral phases used (clinopyroxene, garnet, phengite), or growth zones thereof, were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies. Of several published geothermobarometers, the garnet–clinopyroxene thermometer of Powell [Powell, R., 1985. Regression diagnostics and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited. J. Metamorph. Geol., 3, pp. 231–243.] combined with the garnet–clinopyroxene–phengite barometer after Waters and Martin [Waters, D., Martin, H.N., 1993. The garnet–clinopyroxene–phengite barometer. Terra Abstr., 5, pp. 410–411.] was chosen here, because it provided the most reliable results. Nevertheless, the scatter of P–T-data points for the prograde (stage I), peak metamorphic (stage II), and retrograde (stage III) development of the eclogites is still considerable. Among the many possible reasons for this inconsistency discussed, a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account. Despite the data scatter, an average P–T-path could be estimated, which includes the following coordinates: for stage I: 15 kbar/500°C; 25 kbar/570°C; 32 kbar/650°C; for stage II: 36 kbar/720°C; and for stage III: 24 kbar/680°C and 14 kbar/620°C. This is in fair agreement with P–T-paths derived earlier for other rock types of the BIU on the basis of other geothermobarometers.     

Interplay between erosion and tectonics in the Western Alps

Bedrock fission‐track analysis, high‐resolution petrography and heavy mineral analyses of sediments are used to investigate the relationships between erosion and tectonics in the Western Alps. Along the Aosta Valley cross‐section, exhumation rates based on fission‐track data are higher in the fault‐bounded western block than in the eastern block (0.4–1.5 vs. 0.1–0.3 mm yr⁻¹). Erosion rates based on the analysis of bed‐load in the Dora Baltea drainage display the same pattern and have similar magnitudes in the relative sub‐basins (0.4–0.7 vs. 0.04–0.08 mm yr⁻¹). Results highlight that climate, relief and lithology are not the controlling factors of erosion in the Western Alps. The main driving force behind erosion is instead tectonics that causes the differential upward motion of crustal blocks.     

Contrasting Eoalpine P-T evolutions in the southern Koralpe,Eastern Alps

Summary The Koralpe crystalline complex and the Plankogel unit represent two lithologically distinct units within the Koralpe region of the southeastemmost Austroalpine crystalline basement. The Eoalpine P-T evolution of these two units is derived from new petrographical data. The Plankogel unit and the Koralpe crystalline complex show markedly different P-T evolutions during the early stages of the Eoalpine event. The rocks of the Koralpe crystalline complex experienced eclogite facies conditions with minimum pressures in the range of 15–16 kbar and temperatures in excess of 700°C. At the same time the Plankogel unit resided in a shallower environment at pressures of 10–11 kbar and temperatures of less than 600°C. The tectonic emplacement of the Plankogel unit into its present position on top of the Koralpe crystalline complex took place after the eclogite facies event in a relatively shallow crustal level. After their juxtaposition the Koralpe crystalline complex and the Plankogel unit were affected by a common amphibolite facies metamorphic overprint. The distinctly different P-T evolution during the early stages of the Eoalpine event and a common history at later stages imply that major tectonic processes were operative in this part of the Austroalpine crystalline basement during the Cretaceous. Such processes may have involved subduction of oceanic and continental lithosphere which may have lead to significant crustal shortening within the Austroalpine basement.

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Unterschiedliche Eoalpine P-T Entwicklungen in der Südlichen Koralpe, Ostalpen

Zusammenfassung Das Koralpenkristallin und die Plankogelserie stellen zwei unterschiedliche lithologische Einheiten in der südlichen Koralpe des ostalpinen Kristallins dar. Die P-T Entwicklung dieser beiden Einheiten während der Eoalpinen Metamorphose wurde anhand neuer petrographischer Daten abgeleitet. Das Koralpenkristallin und die Plankogelserie zeigen deutlich unterschiedliche P-T Entwicklungen in einem frühen Stadium der Eoalpinen Metamorphose. Die Gesteine des Koralpenkristallins waren eklogitfaziellen Bedingungen mit Mindestdrucken im Bereich von 15 bis 16 kbar und Temperaturen von über 700°C ausgesetzt. Die Plankogelserie verweilte zur gleichen Zeit in einem relativ seichten Niveau bei Drucken von 10 bis 11 kbar und Temperaturen unterhalb 600°C. Die Platznahme der Plankogelserie in ihrer heutigen Position im tektonisch Hangenden des Koralpenkristallins erfolgte nach dem eklogitfaziellen Ereignis in einem relativ seichten Krustenniveau. Nach ihrer Vereinigung erfuhren die beiden Einheiten eine gemeinsame amphibolitfazielle Überprägung. Die markant unterschiedlichen P-T Entwicklungen in einem frühen Stadium der eoalpinen Orogenese und die gemeinsame Entwicklung in einem späteren Stadium können als Hinweis auf eine umfangreiche tektonische Aktivität in diesem Teil des ostalpinen Grundgebirges in kretazischer Zeit gewertet werden. Diese Tektonik bestand eventuell in einer Subduktion von ozeanischer und kontinentaler Lithosphäre, und kann zu einer signifikanten Krustenverkürzung im Ostalpinen Kristallin geführt haben.

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With 7 Figures     

Paleozoic evolution of the External Crystalline Massifs of the Western Alps

The External Crystalline Massifs (ECMs) of the Alps record, during the Paleozoic, the progressive closure of oceanic domains between Gondwana, Armorica and Avalonia in three contrasting tectonic domains. The eastern one shows the Early Devonian closure of the Central-European Ocean between Armorica and Gondwana along a northwest dipping subduction zone. The western domain is marked by Lower Ordovician rifting followed by Mid-Devonian obduction of the back-arc Chamrousse ophiolite. The central domain underwent Late Devonian to Dinantian extension in a back arc setting associated with southeast dipping subduction of the Saxo-Thuringian Ocean. Based on tectonostratigraphic correlations, we propose that the western domain shows an affinity to the Barrandian domain while the eastern and central domains correspond to the north-eastward extension of the Moldanubian zone, to the south of the present-day Bohemian Massif. From Mid-Carboniferous to Permian, the eastern and central domains of the ECMs, including the internal parts of the Maures Massif, Sardinia and Corsica were stretched towards the south-west along the ca. 1500 km long dextral ECMs shear zone preceding the opening of the Palaeo-Tethys ocean.     

Hf isotope systematics in granitoids from the central and southern Alps

First initial-Hf isotopic compositions for samples from the Alpine domain are presented and discussed. The results are mainly based on zircons and a few whole rocks with ages between 30 and 450 Ma. Of those so far analyzed, the present-day Hf isotopic compositions of zircons from non-metamorphic and metamorphic granitoid rocks vary between 0.2824 and 0.2829. Zircon populations with concordant U-Pb ages have much higher initial ¹⁷⁶Hf/¹⁷⁷Hf than inversely discordant populations which have been contaminated with older zircons containing less radiogenic Hf. Correlated Nd-Hf crustal-residence ages have been found involving model parameters of Hf/Nd=f(Lu/Hf)/f(Sm/Nd) 1.6 for the depleted mantle and f(Lu/Hf)/f(Sm/Nd) 1.2 for elemental fractionations in the crust. The model implies ¹⁷⁶Lu/¹⁷⁷Hf of 0.017 for the bulk crust. It is suggested that the granitoid rocks are the result of mixing of subcontinental mantle-derived magmas with 1.7 Ga old recycled and partially molten crustal material. The continental/mantle component mass-ratio values for the granitoids range between 0.3 and 2.     

Tectonic setting and regional implications of ca 2.2 Ga mafic magmatism in the southern Hamersley Province,Western Australia

Although the presence of post-Hamersley Group mafic intrusive and extrusive rocks in the southern Hamersley province of Western Australia has been known since the area was first mapped in the early 1960s, details of the age, tectonic setting and significance of these rocks have only recently been determined, and are still controversial. These rocks are most commonly interpreted as the products of two temporally distinct periods of continental extension, separated by a hiatus of ～370 m.y. represented by the unconformity at the base of the Wyloo Group. However, the integration of published geochronological and geochemical data with detailed field observations documented in this study shows that ca 2.2 Ga dolerite sills pre- and post-date this unconformity, and were intruded during Ophthalmian orogenesis in a retroarc foreland basin. Furthermore, ca 2.2 Ga mafic magmatism is interpreted to include the Cheela Springs Basalt and is related to subduction beneath the southern Hamersley province, most likely resulting in accretion of part of the Gascoyne Province to the Pilbara Craton.     

Thermal structure of a fossil subduction wedge in the Western Alps

New peak metamorphic temperatures are obtained by Raman spectroscopy of Carbonaceous Material to document the thermal structure of the central Western Alps with high sampling resolution. We show that peak metamorphic T gradually increases eastward from <330 to 350 °C (ultra-Dauphinois to subbriançonnais units), ∼350 to more than 400 °C (Briançonnais domain including the Zone Houillère where metamorphic index minerals are rare) and from 350 to more than 500 °C (Liguro–Piemontese domain). Combined with other constraints on the metamorphic evolution, this dataset reveals a good preservation of the overall thermal structure of the fossil subduction wedge, with no particular thermal overprint during collision. However, local confrontation with P – T estimates and radiometric ages reveals more subtle variations within tectonic units and across the main contacts that are linked to the past activity of the major thrusts and extensional shear zones during subduction and exhumation.     

Composition,age and tectonic setting of amphibolites in the central Bushmanland Group,Western Namaqua Province,southern Africa

The Western Namaqua Province (WNP) of southern Africa is composed of several supracrustal assemblages and intrusive granitoid suites that were formed during the mid-Proterozoic 2.0-1.0 Ga ago. The Bushmanland Group (BG) has been the subject of intense study recently because of its considerable strategic metal potential, hosting several giant stratiform base metal deposits (e.g., Gamsberg and Aggeneys). The BG contains amphibolites which represent metamorphosed tholeiitic basalts with MgO (9-4%) and enriched in Ni relative to modern basalts, a feature reminiscent of Archaean tholeiites. Post depositional mobility of elements is probably widespread and hampers recognition of parental magma composition and source region characteristics. However, the amphibolites have yielded a Sm-Nd isochron age of 1649 ± 90 Ma (ϵ_Nd(T) = −0.48 ± 0.89), which is interpreted as the time of extrusion of the basalt precursors, and by inference a minimum sedimentation age for the underlying metalliferous BG sequence. It follows that the BG sequence evolved during an intermediate period in the history of the Namaqua Province, after major crustal stabilisation around 2.0-1.9 Ga and before the last major tectonothermal event around 1.2-1.1 Ga. Results of a parallel Rb-Sr and U-ThPb study of the BG amphibolites indicate strong metamorphic resetting 1.2-1.1 Ga ago, thereby confirming earlier work on the WNP. Characteristics of the source region to the BG basalts have been inspected with MORB normalised trace element abundances, with the salient result being the BG basalts require derivation from a source that experienced variable geochemical modification similar to that proposed for the mantle wedge overlying modern subduction zones. Such a process is currently preferred over crustal contamination, since the Sm-Nd isochron for the BG amphibolites is remarkably linear, but independent confirmation of the Sm-Nd age is still required before this alternative model can be discounted. To reconcile the requirement of subduction related metasomatism in the source to tholeiitic basalts erupted within what appears to be a cratonic sedimentary basin, we suggest that back are continental extension could provide a satisfactory tectonic setting, as such an environment is intrinsically linked to subduction.     

Palynostratigraphy of some Pleistocene deposits in the Western Alps: A review

In the Alps, interglacial and interstadial deposits are rarely preserved due to the intense erosive effect of glaciers in the valleys. Fortunately, some outcrops and cored sequences located in the field area ranging from Lyon to Evian provided sedimentary profiles datable by palynostratigraphy in a highly documented geomorphological context. An overview of several palynological sequences studied in this large area is proposed, and their position in a general chronostratigraphical pattern is discussed. Particular attention is paid to palynostratigraphical evidence whose relevance is tested with systematic comparisons with long reference European pollen sequences spanning several glacial cycles. Minimum ages are suggested for non-glacial episodes corresponding to the deposits studied.     

Permian high-temperature metamorphism in the Western Alps (NW Italy)

International Journal of Earth Sciences - During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial...     

Coexisting calderite and spessartine garnets in eclogite-facies metacherts of the Western Alps

Summary The coexistence of a colourless and a yellow garnet was observed in eclogite-facies manganese concentrations of the Mesozoic ophiolitic Zermatt-Saas Unit, at the Praborna mine near Saint-Marcel, Val d’Aoste, Italy, and in the upper Maurienne Valley, France. They occur both in oxidised metachert with hematite and braunite (+ minor Mn-pyroxenoid and tirodite, rare tiragalloite; with ardennite or piemontite in distinct layers), and in more reduced, carbonate-rich boudins included in it. The co-occurrence takes a variety of textural aspects, from coexisting euhedral garnets (10–100 μm in size for the calderite to mm-size for spessartine) to sharp overgrowths of yellow calderitic garnet on colourless spessartine, to yellow cauliflower-like masses (a few hundreds of μm in size) overgrowing colourless spessartine and showing evidence of oscillatory zoning, resorption stages and resumed growth. Sector zoning and anisotropy are common, although not consistent features. Compositions can be expressed to 95% in the quadrilateral system (Ca, Mn²⁺)₃ (Al, Fe³⁺)₂ Si₃O₁₂, with less than 1.0 wt% MgO and 0.8 wt% TiO₂ in colourless spessartine, and less than 0.2 wt% MgO and 1.6 wt% TiO₂ in yellow garnet. Calcium partitions into the ferric garnet. Coexisting pairs define two compositional gaps, bounded by values of the Fe³⁺/(Al + Fe³⁺) ratio of 10 and 15% for the first one, of 40 and 65% for the other. The optically obvious discontinuity (colour change and Becke’s line) corresponds to the narrower gap, between colourless spessartine and yellow spessartine, whereas the broad compositional gap occurs within yellow garnet, between yellow spessartine and yellow calderite, and is only revealed by back-scattered electron images. Only the latter can be a candidate for a miscibility gap, if any. Present address: Centre de Géochimie de la Surface – EOST, 1 rue Blessig, 67083 Strasbourg Cedex, France     

Structural analysis of sheath-folds in a meta-chert from the Western Italian Alps

A detailed meso- and microscopic structural investigation of a laminated manganiferous meta-chert from the Western Italian Alps has resulted in the recognition of five deformation phases. During the third phase large subhorizontal shear movements took place, resulting in reorientation of pre-existing structures and sheath-fold formation. This was accompanied by a decrease in pressure, reflected by the zoning of blue-amphiboles and by microboudinage and the formation of stretching cracks in minerals. The orientation of amphiboles, together with some evidence from quartz c-axis fabrics suggest that the deformation took place by simple shear. During the late stages of sheath-fold formation the deformation became non-rotational.     

Geological setting, mineralogy, geochemistry and genesis of the Middle Archaean Kalyadi copper deposit, western Dharwar craton, southern India

The Kalyadi polymetallic copper deposit occurs within the Middle Archaean (≥3.0 Ga), medium-grade Kalyadi schist belt which consists predominantly of ultramafic-mafic schists interbedded with chemogenic chert, detrital high Al-Mg schists and siliceous schists. This sedimentary exhalative type (SEDEX type) ore-body is the only copper deposit hosted in cherts in the western Dharwar craton. The Kalyadi supracrustal rocks are intruded by tonalite-trondhjemitic gneisses (ca. 3.0 Ga) and granite (ca. 2.6 Ga). The Kalyadi copper deposit is polygenetic in nature. The primary ores represented by disseminations of pyrite ± linneite and chalcopyrite ± magnetite essentially along the bedding lamination of the metachert are referred to as the metamorphosed chert-sulphide rhythmites of a primary stratiform type. The ore is of low-grade and records imprints of at least two events of deformation. Pyrite is characterised by high-Co values (262–4524 ppm) and high–Co/Ni ratios (3.0–19.7). Rare earth element patterns of the primary ores and the host metacherts are identical, characterised by La enrichment, absence of Eu anomalies and flat to depleted HREE patterns with δ ³⁴ S = −0.8‰. The secondary (remobilised) ores are structurally controlled occurring as veins and stringers discordant to the bedding lamination or schistosity. The constituent ores are chalcopyrite-pyrite-pyrrhotite with minor pentlandite. These sulphides with low-Co/Ni ratios (0.87–1.80), have either a strong positive or negative Eu anomaly and show slight HREE enrichment. The δ ³⁴ S value ranges from +2.64 to −4.29‰. It is interpreted that the primary stratiform ores and the cherts were derived from volcanogenic hydrothermal fluids as syngenetic/chemical deposits in a deep sea environment. The secondary epigenetic mineralisation is related to subsequent migmatisation, deformational events and granitic activity. Received: 8 September 1995 / Accepted: 18 November 1996